Applicationsof SurfaceScience2 (1979)543—557© North-HollandPublishingCompany

INFRARED REFLECTION STUDY OF HYDROCARBON CHEMISORPTIONON EVAPORATED NICKEL AND PALLADIUM FILMS

M. ITO, Y. MORI, T. KATO and W. SUETAKALaboratoryof InterfaceScienceofMetals, FacultyofEngineering,Tohoku University,Aramaki, Sendai980, Japan

Received2 May 1978Revisedmanuscriptreceived 18 July 1978

The chemisorptionof C2H2, C2H4 on Ni and Pd films depositedatroom temperaturewas

investigatedby meansof infrared reflectionspectroscopy.At roomtemperature,acetyleneandethyleneare chemisorbedin a C2~,,structurewith theC—C bondsparallelto themetalsurface.Thehybridizationchangeof thecarbonatoms from sp towardsp

2 with acetylenechemisorptionand from sp2 towardsp3 with ethylenechemisorptionseemsto occurat roomtemperature.Astrangefeatureof the spectrawasan abnormallylow C—H stretchingfrequency,which will bediscussed.

1. Introduction

Infrared reflection spectroscopycanproducedirect information aboutthe geo-metric arrangementof an adsorbedspeciesand its orientationwith respectto a sur-face [11. The majority of infraredstudies,however,haveinvolved infraredtrans-parentmaterialssuchas silica or aluminaunder relatively poor vaccum conditions[2], and it is only recentlythat the high sensitivity reflection spectroscopytech-nique hasbeensuccessfullyappliedto the studyof COchemisorbedonseveralmetalsurfacesunder UHV conditions [3, 4] . However, the reflection studiesso far re-portedhave dealt almost exclusivelywith CO chemisorption,probablybecausead-sorbed CO showsan exceptionallyintenseabsorptionandinfraredbandsof the ad-sorbedspecieslie outsidethe atmosphericwater vapourregion.

The identification of the chemicalnatureandbondingin hydrocarbonschemi-sorbed on a transition metal surface is fundamentallyimportant to understandcatalyticreactionson surfaces.In previousinfraredtransmissionstudiesof acetyleneand ethylenechemisorptionon supportedmetalsurfaces,the predominantsurfacespecieswere saturatedhydrocarbons,althoughsomeunsaturatedspecieswere iden-tified [5] Recentlow temperatureinfrared transmissionwork on ethylenechemi-sorbedon aluminasupportedPd andPt surfaces,and also roomtemperatureworkon silica supportedPt, haveindicatedtheexistenceof ir-complexedethylenicspecies

544 M Ito etal. I Infraredreflectionstudyofhydrocarbonschemisorbedon Ni andPt

[6, 7]. However,the experimentalconditionsin theseinvestigations,i.e. highamb-bient pressureand a poorly definedsurfaceprecludea directcomparisonwith therecentresultsfor well definedsurfacesunderUHV conditions.A recentLEED in-tensityanalysisof acetyleneadsorbedonPt(l 11)revealedthatchemisorbedacetylenelies parallelto the planeof the surface [8]. Ultraviolet photoemissionstudies[9]indicatedacetylenechemisorbedon Pd(l11) andPt(l11)asbothir-bonded,at lowtemperatures,anddi-a-like olefmic surfacecomplexesathightemperatures.Further-more,high resolutionelectron energyloss spectroscopy(ELS) [10] hasbeenusedto measurethe surfacevibrationsof hydrocarbonsadsorbedon cleanmetalsurfaces,and since the surfaceselectionrules for ELS and infraredreflection spectroscopyare thesame,a comparisonof the techniqueswill be of interest.

We have previouslyreportedinfraredreflection results[111 for ethyleneadsorp-tion on evaporatedPt and Ag surfacesandindicatedthat acetylenicspeciesare pro-ducedby a dehydrogenationreactionon the surface.In this paperwe will reportin-fraredreflection spectraof acetyleneand ethylenechemisorbedon Ni andPd poly-crystallinefilms andcomparetheresultswith thoseof UPS and ELS.

2. Experimental

The UHV IR cell and full experimentalprocedureswill be describedelsewhere[12]. The spectrometer(IR-G, NthonbunkoLtd.)was operatedin the doublebeammode.Repetitive scanningof the spectrumandcomputeraveragingwere necessaryto obtaina reasonablesignalto noise ratio.

Ni and Pdwere evaporatedontopolishedquartzglassby resistivelyheatingwires(3N, 1 mm Ni, 4N, 1 mm Pd, Wako Chem.Co., Ltd.). Backgroundpressuresin thelow l0~ Torr range could be routinely obtained,although the pressureroseto6 X l0~ Torr during film deposition. Acetyleneand ethylenegases(high purityreagentgas, 99%, TakachthoCo., Ltd.) were used.Thermaldesorptionwas moni-toredby useof a quadrupolemassspectrometer(NEVA-5 15), which alsoservedasanindication of theresidualgasesin thevacuumchamber.

3. Results

3.1. Acetylenechemisorption

The reflection spectraof acetylenechemisorbedon Ni andPd films are shown infig. 1, for the C—H(D) stretchingregion, 3400—2600 cm~(2700—2300cm—i)and in fig. 2 for the C—H(D) bendingregion, 1000—700cm1 (700—550cm~).Thevaluesin parenthesesarethe correspondingwave numbersfordeuteroacetylene.Two bandswere observedineachregionwithcomparableintensities.At roomtern-

M. Ito etal. / Infraredreflectionstudyof hydrocarbonschemisorbedonNi andPt 545

z

3 )0 3200 3000 2600 2600WAVENUMBER (cml

(a) C2H,/ NI

~ (b) C,Ht/ Pd

Cc) C2D2/ NI

(d) Cd) C2D,/ Pd

IOL

2700 2500 2300 T — 300 K

WAVENUMBER (cm~)

Fig. 1. Infrared reflection spectraof chemisorbedacetyleneon evaporatedNi andPd films atroom temperature.C—H stretchingregion.

-J (b)

950 900 850 800 750WAVENUMBER (cm~)

(a) C2H,/NI

~ ~flci CD) C1111 / Pd

— Cc) C~D./NICd) Cd) C,D,/Pd

IOL6~0 600 550

WAVENUMBER (cm’) I — 300 K

Fig. 2. Infrared reflection spectraof chemisorbedacetyleneon evaporatedNi andPd films atroom temperature.C—H bendingregion.

546 M. Ito etal. / Infrared reflection studyof hydrocarbonschemisorbedon Ni and Pt

perature these bands increased in intensity until 5 L admissionof acetylene,and

any further addition of gasleft the intensity unaltered.Largepeakswere observedat 850 crn1 (650 cm~)and 770 cm~(585 cm—1)on the Ni film. We haveas-signed thesebands to the C—H(D) bendingvibrations of chemisorbedacetylene,since as in the gas phasespectrum,wherethe C—H out-of-planebendingvibrationoccursat 729 cm~(539 cm~),thebandsare themost intensein thespectrum,andarealso relatively nearto this value.

Adsorptionon Pd films also gave rise to strongbandsdue to the C—H bendingvibration at around835 cm~(630 cm~)and755 cm~(575 cm—1). The strongintensity of thesebandssuggeststhatchemisorbedacetylenemoleculesareorientedwith the C~Caxis parallel to the metal surfacesincethis reflection techniqueonlystimulates vibrations having dipole moment changes perpendicularto themetal sur-face [13].

Many bands appeared in theC—H stretchingregion and thesebands were assignedto C—Hstretchingbandsof unsaturatedhydrocarbonspecies(above 3100 cm—’)and saturatedhydrocarbonspecies(below 3000 cm1) as describedbelow. Freeacetylenehas fundamentalvibrations of the symmetricand antisyrnmetricC—Hstretching at 3324 cni~ (2700 cm~),v

1(~~),and 3287 cni~ (2427 cm1),

~

3(~U), respectively [14]. The infrared active antisymmetricC—H stretchingbandof molecules of this orientation should not appearsince the direction of the dipolemoment change of this vibration is parallel to thesurface,if the surface is smooth.Of particular interest was the observationof absorptionbandsassignableto sym-metric C—H stretchingvibrations in the reflection spectraas shown in fig. 1. Ac-cording to the selectionrule, thesebandsareforbiddenin theinfrared spectrumofthe free molecule. Their appearancein the reflection spectrumindicatesa loweringin the symmetryof themolecule,and, therefore,theadsorbedacetyleneis distortedfrom thelinear structure.For suchan orientationof distortedmolecules,therefore,a symmetricC—H(D) stretchingvibration bandshould appearin the reflectionspec-trum becausethis vibration gives rise to an oscillating dipole normal to themetal

surface.Two kinds of absorptionarealso locatedin this region: 3250, 3150 cm~

(2525, 2450 cm~)on Ni films and 3350, 3220 cm1 (2625, 2510 cm~)onPd

films. The lower frequency band on both surfaces shifted to lower frequencies by50—SO cm—1, and increasedin intensity when the temperature of the films wasincreased to 400 K. All bands assignable to acetylene species are shown in table 1.

Other bandsappearedin the low frequencyC—H region(2600—2800cm1) andare also shown in fig. 1. This is the first infraredobservationof C—H stretchingbands in this region. On Ni films a band at 2750 cm~appeared along with a bandat 2900 cm—1, whereas on Pd films, bands at 2870 cm~and 2750 cm~were ob-served. These bands appeared immediately after room temperatureadmissionofacetylene, and were relatively smaller in intensity than the C—Hbands of the acety-lene species, confirming that the latteris the dominantsurfacespecies.

M. Ito etal. / InfraredreflectionstudyofhydrocarbonschemisorbedonNi andPt 547

Table 1The observedandcalculatedfrequenciesfor chemisorbedacetylenefrom thevalenceforce fieldshown in table 4

Vibrational modes Species I on Ni SpeciesII on Ni

C2H2 C21)2 C2H2 C2D2

obs. calc. obs. calc. obs. calc. obs. calc.

A1 CHsym.str. 3150 3150 2450 2444 3250 3247 2525 2516CCstr. 1800 1804 1660 1663 1845 1847 1705 1704HCCdef. 850 850 650 639 770 771 585 586CMe. sym. str. (495) 477 447 (460) 461 426CCMe. def. 45 45 41 41

B2 CHasym.str. 3102 2279 3198 2349HCCdef. 980 831 891 764CMe. asym. str. 523 443 492 412CCMe.def. 195 191 180 176

Species I on Pd Species II on Pd

C2H2 C2D2 C2H2 C2J)2

obs. calc. obs. calc. obs. calc. obs. calc.

A1 CHsym.str. 3220 3225 2510 2504 3350 3340 2625 2584CCstr. 1850 1853 1710 1707 1880 1882 1730 1740HCCdef. 835 834 630 627 755 753 575 573CMe.sym.str. (480) 470 440 (455) 453 418CCMe.def. 44 44 40 40

B2 CHasym.str. 3175 2332 3292 2417

HCCdef. 961 816 870 747CMe. asym.str. 514 435 482 403CCMe.def. 192 188 176 172

3.2. Ethylenechemisorption

The infrared reflectionspectraof ethylenechemisorbedon evaporatedNi and Pdsurfacesat room temperaturewere very similar to the previousresults on Pt [11]andare depictedin figs. 3 and4 showingthe C—H stretchingandC—H bendingre-gions respectively.A strong,broadbandappearedat around1000 cm~, dueto theC—H bendingvibration, its largeintensityindicatingthat thechemisorbedethylenemoleculesare orientedwith the C—C bond parallelto the smoothmetal surface.There is also evidenceof an acetylenicspeciesexisting on theNi surface,by virtueof a band at 850cm’ which wasassignedto an acetylenicC—H bendingvibration[15].

The bandsappearingin the C—H stretchingregion (fig. 3) were assignedas fol-lows: acetylenicspecies(~32O0cm

1),ethylenicspecies(~u29O0cm1), saturated

548 M. Ito et al. /Infraredreflectionstudyofhydrocarbonschemisorbedon Ni andPt

I I I I I

~Ni

OL

T 300K

I I I I I

3400 3300 3200 3100 3000 2900 2800 2700 2600

WAVENUMBER (cm’

Fig. 3. Infrared reflection spectra of chemisorbed ethylene on evaporatedNi andPd films atroomtemperature.C—H stretchingregion.

hydrocarbon species (~275O cm—1). On Ni these bands appeared at 3250, 3075,

2880 and 2700 cm~, whereas on Pd they occurred at 3300, 3200, 2950 and2750 cm~. The bands began to appear after admission of 0.5 L of gas, and satu-ratedin intensityafteran exposureof 2 L.

-

10 L

1 300K

I I I I

1040 1020 1000 980 960

WAVENUMBER (cm~)

Fig. 4. Infrared reflection spectra of chemisorbed ethylene on evaporated Ni and Pd films at

room temperature.C—Hout-of-plane vibration.

M. Ito eta!. / Infrared reflection studyofhydrocarbonschemisorbedon Ni andPt 549

The bandsaround3200 cm—1 may be due to anacetylenicspeciesadsorbedonthe surface, since they agree well with the frequenciesmentionedin section 3.1,andalso with thosereportedby usin a previousstudyof acetyleneadsorbedon anevaporatedPt film [11].

Further evidencefor the assignmentof thesebandscomesfrom a comparisonofa recentreflection infrared studyof ethyleneadsorbedon a Pt foil [15], and thebehaviour of acetylenic species as shown by UPS [9] . Theinfraredspectrumshowedbandsat 3200 cm~and 850 cm—1 only at temperaturesabove200 K, which isknownby UPSto be the temperatureat which acetylenicspeciesoccur.

Low frequencyC—H stretchingbandswhich havealreadybeenreportedfor thefirst time by us [11], were also observedon Ni (2700cm1) andPd(2750cm~)films respectively.The band on Pt occurredat 2730 cm—1 andwas, unlike thebandson Ni andPd, more intensethan the acetylenicC—H stretchingbands[11].

3.3. EthanechemisoiptiononPdand Ptfilms

The chemisorptionof ethaneon Pd andPt films was studiedat roomtemperaturein orderto assigntheunusuallylow frequencyC—Hstretchingbandswhichoccurredupon acetylenic and ethylenic adsorption.On Pd film surfacebandsat 2850 and2700 cm1 were observedimmediatelyafter 10 L admissionof ethane.Also on Ptfilms low frequencybandsat 2750and2600cm’ appeared(fig.5).Thelatterbandwasweakin intensityandshifted to 2650cm—1 at 350 K.

A careful searchof the regionbetween1600and 1100 cm~was madeon NiandPd films in orderto observethe CssC,C=CandCH

2 scissoringabsorptionbands

____________________________ Pd

z0Cl) _______________________ Pt

-~-~ II.,.4

1300K

3000 2900 2800 2700 2600 2500

WAVENUMBER C crn1 )

Fig. 5. Infraredreflection spectraof chemisorbedethaneon evaporatedPd andPt films at roomtemperature.C—H stretchingregion.

550 M. Ito eta!. /Infrared reflection study ofhydrocarbonschemisorbedon Ni andPt

althoughthis was hamperedby atmosphericwater vapourbands.Somevery weakbandsaround1400and1200cm—1 wereobservedabovethebackgroundnoiselevel.

4. Discussion

4.1. Acetylenechemisorption

All the observedbandsare paired,indicating the existenceof two kinds of ad-sorbed specieson the surface.We refer to the two speciesas I and II tentatively.SpeciesI gives largeshifts of the bandsfrom thefree molecule,thebandsbeinglo-catedbetweengaseousacetyleneand ethylenefrequencies,andspeciesII shiftsonlyslightly from the free acetylenefrequencies.The former, I, seemsto be a stronglychemisorbedspeciesandthe latter, II, weakly adsorbed;however,bothspeciesap-pearedsimultaneouslyat the initial stageof adsorptionandalso resistedupon de-sorption,suggestingthesespeciesoccupydifferent surfacesites.

Conventionalinfrared transmissionmeasurementsof supportedmetal particleshave beenlimited by the low frequencycut-off of the supportmaterial(usuallyaluminaor silica). However,with the infraredreflection technique,thereis no suchlimitation, andwe havebeenable to observebandsdue to theC—H bendingvibra-tions of chemisorbedhydrocarbonswith ease.In the C—H bending region,bandsoccurredat 850 cm1 and835 cm’ in thecaseof Ni and Pd respectively,showinglarger intensitiesthan the other bandswhich occurredat 770 cm~(on Ni) and755 cm~(on Pd),which were assignedto speciesII, the bandsbeing nearthe gasphasevalueof 729 cm—1. This intensity relationshipwasalso observedin the C—Hstretching region, where the banddue to thestronglychemisorbedspecies,in otherwords, the one showinga largefrequencyshift from the gas phasevaluewasmoreintensethan theweakly adsorbedspecies,asshown in fig. 1.

A bent structureof acetylenehas been determinedin many transitionmetalcomplexesby X-ray analysis[161. Furthermore,the infraredspectraof coordinatedacetylene,which are similar to that of electronicallyexcitedacetylene,havebeenelucidatedassuminga bent structureof acetylene[17]. It is, therefore,likely thatthe chemisorbedacetylenespecieshas a bent structureof C

2~symmetrydistortedfrom the linearD~hstructure,becauseall theobservedbandsare of the A1 type ofthe C2v molecule(table 1).

A recent UPS studyof acetylenechemisorbedon single crystallineNi(l 11) re-vealed molecularly chemisorbed acetylene [18], and the existenceof surfaceacti-vated complexes on the Pd and Pt surfaces hasbeenreportedat elevated tempera-

tures [9], although not on Ni surfaces.However, the bandsdue to chemisorbedacetylene on the Ni surface displayed a larger frequencyshift from the gas phasevalue in our spectrathan on the Pd surface,which suggestthat a greaterrehybridi-zation occurredon Ni surfacesthan in the case of adsorptionon Pd at thecarbonatomsfrom sptowardsp

2 forming di-a-likebridgebonding.

M. Ito etal. /Infrared reflection studyofhydrocarbonscheinisorbedon Ni andPt 551

The chemisorbedacetylenespeciesare stable sincethey could be producedon asurfacepretreatedby hydrogen,whereafterhydrogenationwould not occur.How.ever, initial dissociationmay occur: absorptionbandswere observedat 2750cm~(on Ni andPd),and sincethe free CH radicalgivesanabsorptionbandat2729cm—1[14] the bandmaycorrespondto an M~C—Hspecies.A recentUPS studysupportsthe possibility of dissociationof acetyleneto adsorbedCH radicalsat room temper-atures [18]. There may be an alternativeexplanationfor theselow frequencyC—Hbandsdescribedbelow.

A massspectroscopicanalysisof theresidualgasin thevacuumchamberrevealedthat more than 90% was hydrogen. Hence, the evaporatedfilms were coveredtosome extent with active hydrogen, also shown by an intensehydrogenthermalde-sorption peak. This means that a hydrogenationreactioncould occur on thesehy-drogenenrichedsurfaces.Thus,adsorbedacetyleneorethylenecould be convertedto saturatedhydrocarbonspeciesto yield similar speciesto those obtainedbyethane adsorption.

Transmission infrared studies of acetylene on silica supported Pd displayed bands

at 2910 and2850 cm1 dueto C—H stretchingof eitherethyl radicalsor polymer-ized species[19] which were the productsof hydrogenationreactionsof acetyleneon the surfaces.Therefore,the bands at around 2900 cm’ on Ni films and2870 cm—1on Pd films are indicative of paraffinic hydrocarbon species. Thus, theunusuallylow frequencyband,at2750cm~on both metalsurfaces, which appeared

in this study might be assignable to eitherthe abovementionedC—H radicalspeciesor anotherspecieswhich interacts strongly withthe metal atomsasdescribedbelow.

4.2. Ethylenechemisorption

Ethyleneadsorptionon evaporatedNi and Pd surfacesproducedthesameresultsas on evaporatedPt surfaces,previouslystudied [11]. At roomtemperaturedehy-drogenationof ethyleneoccursproducingan acetylenicspecieswhich is thedomi-nant species.The acetylenic speciesthus producedwas the sameas for acetyleneadsorption,showinga close frequencycorrespondence.This speciesonly occurredalongwith ethylenespeciesat roomtemperature,andnot at all below200 K [15].

Ethylenedehydrogenationto acetylenehasbeenreportedby many investigators.LEED and UPS results support the conversionof ethyleneto acetyleneat roomtemperatureon Ni andPt single crystalsurfaces[20, 26,27]. In thesepapersit isreportedthe ethylenechemisorbsmolecularlyon themetalsurfaceat low tempera-ture.The resultsare in goodaccordwith our presentresults.

The fact that thoseacetylenicspeciesappearedupon adsorptionevenafter 100Lof hydrogenadmissionaccompanyingdesorptionof alargeamountof hydrogen,in-dicatesthe adsorbedethylenedispelled the chemisorbedhydrogenand the dehy-drogenationreactionstill occurredon the hydrogenenrichedsurfaces.

The frequenciesof the C—H stretchingandC—H out of plane bending vibrationsof the ethylenicspecieswere similar to thoseof Zeise’ssalt [21]. l’his indicates that

552 M. Ito etal. IInfrared reflection study of hydrocarbonschemisorbedon Ni andPr

thesespeciesare ir bonded,in agreementwith recentUPS [9] and transmission in-fraredresults[6, 7].

The low frequencyC—H stretchingbandsappearedwhen ethylenewas intro-ducedon the PdandNi films at roomtemperature.The routeof thehydrogenationreaction occurring on these surfacesis probably by the samemechanismsas des-cribed for acetylenechemisorption.Therefore,it is of greatinterestthat both thehydrogenationand the dehydrogenationreaction of ethyleneto saturatedhydro-carbons or acetylenicspeciesoccurredsimultaneouslyon the samesurface.Thesedisproportionationreactions(2C2H4 -÷ C2H2 + C2H6) have alredybeenstudiedonseveralmetalcatalystsby using variousmethods.

4.3. EthaneadsorptiononPd

Acetyleneandethyleneadmissionon Pd surfaceproducedhydrocarbonspecieswhich showed an extraordinarily low frequency band in the C—H region at2750 cm’. Ethaneadsorptionon Pd films displayedsimilar low frequencybandsat 2850and 2700 cm’. Theselow frequencybandswere also observedon Ag andPt films [11]. As wasdiscussedbefore, the lowest frequencybandfor eachmetal istoo low to be assignedto the C—H stretchingmodeof paraffinic hydrocarbonspe-cies which normally occursaround2910and 2850cm~on Pd. However,the bandat 2850 cm~on Pd surfacesis indicative of thepresenceof sucha saturatedtypespecies.

Some coordinated compounds containing a very strong interaction between an

aliphatic C—Hgroup and a transitionmetalatomhaverecentlybeenreported [22].According to this paper,a hydrogenattachedto carbonwould interactwith a metalatom and the C—H bond will be stretchedand is associatedwith a strong C—Hstretchingbandat the unusuallylow freequencyrangebetween2600—2800cm~.This type of interactionon themetalsurfacehasnotbeenobservedbefore,and theresult may also be importantin relationto the attemptsto devisepracticalcatalystsfor crackingandreforming aliphatic hydrocarbons.It is apparentthat furtherworkis necessaryto obtainanunambigousexplanationof thelow frequencyC—H stretch-ing bands.

4.4. Normalvibrational analysis

It is interesting to estimate themoleculargeometryandvibrationalforce con-stantsof chemisorbedacetylenefrom theobservedfrequencies.Therefore,a simplenormal coordinatetreatmenthasbeenadoptedfor the distortedC2~structureof di-o (I) andthe ir-adsorbedmodel(II) (fig. 6) in order to obtain furthersupportfor theinfrared band assignmentsand the structuraldistortions of adsorbedacetyleneonmetal surfaces.

Gand F matriceswere constructed for both models according to Wilson’s methods[23]. In thesemodels,out-of-planevibrations wereexcludedfrom thecalculations

M. Ito eta!. /Infraredreflectionstudyof hydrocarbonschernisorbedon Ni andPt 553

$28 1.28

(I) (n)

Fig. 6. Assumedmodelscorrespondingto thedi-a (I) and thea-adsorbed(II) species.

becausethey do not influencethe in-planeC~C,C—H, C—metalstretching and HCCbendingmodeswhich are of prime interest.Severalmoleculargeometrysetsof theC~C,C—H andC—metalbonddistancesare shown in table2. In eachcalculationHCC bondangleswerevariedfrom 180°to 120°instepsof5°,sinceseveraldifferentvaluesof these parameters have been observed in various metal complexes. How-ever, calculationsbasedon thesesetsof bondlengthsfor the chemisorbedspeciesand for the free moleculeshowedthat thechoice of theseparametershasaninsigni-ficanteffect on thefinal results.Henceall calculationspresentedherewereperformedusing a single set of bond lengths(TCC = 1.284, rCH = 1.074,rCM = 2.25 A). The

assumedmodelsareillustratedin fig. 6 alongwith therelevantparameters.TheNi—Cbond length and Ni—Ni distancewere takento be 2.25 and2.49 A, respectively,basedon a recentLEED intensityanalysis[8] of acetyleneon Pt (Ill).

Since the generalharmonicforce field for suchadsorbedmodelshasmanymoreparametersthanthe observedfrequencies,theforceconstantscannotbedetermined

Table2Thesetsof geometricalparametersusedin thecalculationsa

r~c rC_Me. LCCMe.b LCCMe)~ LCMe.Cc

free acetylene 1.0605d 1.2033d 2.30 ~ 106.2 74.9 30.3excitedacetylene 1.0800 l.

388e 2.oof 106.0 69.7 40.6estimatedvalues 1.o74g 1.284g 2.25f 105.5 73.4 33.2

a Distancesin A, anglesin degrees.b Derivedfrom assumeddi-a model usingC—Me.,C~Cand Me.—Me.distances.c Derivedfrom assumedsr-adsorbedmodelusingC—Me.,~C and Me.—Me. distances.d Seeref. [251.e See ref. [171.

f Assumedarbitrarily.g Approximatevaluesfound by linearly interpolatingbetweenthoseparametersof acetylene

andethylenefrom observedfrequencies.

554 M. Ito etaL / Infraredreflection studyof hydrocarbonschemisorbedon Ni and Pt

Table3Symmetrycoordinatesused in the calculations a

A1 species

CH sym. str. 51 = .A(r1 +CC str. S2 = MCMe.str. S3 = I~(pi+p2)/2~

2HCC def. S

4 = ~(0~ +CCMe.def. S5 = ~ + 02)/21/2

B2 species

CH asym.str. S1 = ~(rj —

CMe.asym.str. 52 = ~(m—p2)12”2

HCCdef. S3 = ~(°~ — 02)/21/2

CCMe. def. S4 = ~(Oi — 02)121/2

a Definition of internal coordinates,ri, CH bonds,d, CC bond,Pi, CMe. bonds,0~,HCC angles,

Oj, CCMe.angles.

uniquely,andall the off-diagonalterms of the matriceswere,therefore,neglected.To obtainreasonableforceconstants,an iterativeprocesswasadopteduntil thebestfit betweenthe observedand calculatedacetylenefrequencieswas obtained.Thevaluesof the sets of structuralparametersand the symmetrycoordinatesusedinthecalculationare listedin table 2 and3, respectively. Since two kindsof acetylenicspecieswere found (speciesI and II) on bothmetals,two setsof force constantswere obtained as shown in table4. However, a calculationfor model II using thesameforce constantsas for model I gavealmostthe samefrequenciesin C—H andC~Cstretching,except that slight increases (10 cm—

1) were found in C—metalstretchingand HCC bending modes. Hence, we could not conclude with a reason-able model for the di-o bridged (model I) and the ir-adsorbed(model II) because

Table4Valuesof forceconstantsadjustedto fit theobservedfrequenciesa

KCH KCC KCMe. - HHCC - - - —~

species Ion Ni 5.23 12.33 1.55 0.232species lIon Ni 5.56 12.97 1.45 0.189speciesI on Pd 5.48 13.04 1.50 0.223species lIon Pd 5.89 13.48 1.40 0.180

freeacetyleneb 5.95 15.79freeethyleneb 5.08 9.57

a K in mdyn/A,H in mdyn A.bSeeref [141

M. Ito etal. / Infrared reflectionstudyof hydrocarbonschemisorbedonNi andPt 555

both models faithfully produced the same shifts. The calculatedfrequenciesofdeuteroacetyleneusing the sameforceconstantsfor acetylenegavecloseagreementwith experimentallyobservedfrequenciesas shown in table 1, thusjustifying theassignmentsandthe modelsused.

The calculatedfrequenciesare moresensitiveto the HCC bond angle. The bestfit occurredat near 140°and this value is in good agreementwith that of coordi-

natedacetylenedeterminedby X-ray analysis [16] ,althoughthevaluethusobtainedmay not be so reliablebecauseof grossassumptionsin the calculations.The finalvaluesof the force constantsarelistedin table4.

Iwash.itahas reportedthe existenceof coordinatedacetyleneon cobaltcarbonyland demonstrateda similarity of the vibrational spectrumof complexedacetyleneto that of electronicallyexcitedacetylene.In thiscomplex,thefrequencyoftheC~Cstretchingvibrationwas found to be lower thanthat of free acetyleneby 570cm—1.This implied a largedistortionof the coordinatedacetylenefrom its free state.Themoleculargeometriesof the acetyleneligandsin many transitionmetalcoordinatedcomplexeshave beendeterminedfrom X-ray analysisof their crystal structures.The averageC—C distanceis 1 .28 A and theligand isbent synmietrica.llyat carbonatomsto give HCC bond anglesof 130°—140°.Anderson[24] hasalso derivedthestructuraldistortionsof acetyleneadsorbedonNi (Ill) by amolecularorbitalstudy.He suggesteddi-a bridgebondingis a preferablestructurewhich showsdistortionssimilar to those in coordination compounds.

As shownin table 4, the C~Cstretchingforce constants,12.3—13.5mdyn/A, aresignificantly smaller than that of free acetylene,but largerthan thevaluesof freeethylene(9.57 mdyn/A) or coordinatedacetylenein Co complexes(6.18mdyn/A)[17]. Although a direct comparisonis very difficult becausedifferent forcefieldswere adopted,the obtainedforce constantssuggestthe ~C bond lost some of its

triple bond characterin the adsorbedspecies.Theseresultsare in good agreementwith UPS andELS results.

4.5. Thermaldesorption

Since exposure of acetylene to clean metal surfaces leads to the formation of avariety of adsorbates,the thermal desorptionspectrumwas examinedto confirmthespeciesdeducedfrom theIR spectrum.Thedesorptioncharacteristicsofhydrogenfrom Ni films are shown in fig. 7. The measurementswere madeafterexposuretoacetyleneat I X l0~ Torr for 10 mm and subsequentevacuationof the cell to1 X 10—8 Torr.

The main desorptionpeakwas due to hydrogen.Two hydrogenpeakswerede-tected,one peakat around 120°Canda much smallersecondpeakaround210°C.Very similar spectraof hydrogendesorptionwere obtainedfrom Pd films after ex-

posureof acetylene,but the peakmaximumshifted to a lower temperaturefrom120°Cto 100°C.The correspondingdesorptionpeaksof massnumber4 (D

2) wereobserved by introducing deuteroacetylene to the cell.

556 M. Ito eta!. /Infrared reflection studyofhydrocarbonschemisorbedon Ni andPt

C2H2/NI

300 350 400 450 500

TEMPERATURE K

Fig. 7. Thermaldesorptionspectraof adsorbedhydrogenon evaporatedNi films.

A previousstudy indicates that acetyleneis adsorbedon Ni and Pd films bymeansof C—H bondbreakingresultingin theproductionof adsorbedhydrogenandsurfacecarbide.Therefore,the first strongpeakat 120°CprobablyoriginatesfromC—Hbondbreaking(dissociatedspecies)becausethe samedesorptioncurvefor massnumber4 wasobservedby introducingdeuteroacetyleneto the cell.

The infraredC—H stretchingand C—H bendingabsorptionbandsdid not reducein intensity in thesetemperatureranges(30°—200°C).This factindicatesthatonlysomeof the surfaceacetylenicspeciesdissociatesandthat mostof the acetylenere-mainsunchangedon the surfaceat temperatureslower than 200°C.Therefore,it isprobablethat the observedpeakat 120°Coriginatesfrom hydrogenformedin thedissociationof acetylene.However,we haveto notethat the surfacesbeforeadmis-sion of gas havealreadybeenexposedto residualhydrogen.Thus part of the hy-drogendesorptionmaybe contributedto this adsorbedhydrogen.

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